1. Field of the Invention
The present invention relates to a hydrostatic thrust bearing. The thrust bearing has at least two pairs of pressure surfaces to which oil can be applied and which are arranged spaced-apart one behind the other in the direction of load application. Each pair of pressure surfaces includes two pressure surfaces which are located frontally opposite each other, wherein one of the pressure surfaces is provided on an end face of an outwardly directed radial flange of a bearing bushing mounted on a shaft, while the other of the pressure surfaces is located on the end face of an inwardly directed radial flange of a support bushing on an abutment side. The bearing bushings are provided with axial spacer sleeves which are directed in one direction and the support bushings are provided with axial spacer sleeves which are directed in the opposite direction.
2. Description of the Related Art
Hydrostatic thrust bearings have a high load-bearing capacity and provide an operation which is almost entirely free of wear. In addition, contrary to hydrodynamic bearings, thrust bearings make possible an operation at low rates of rotation while the friction is negligibly small. Additional important features are high stiffness and low play.
Although hydrostatic thrust bearings must be supplied with pressure oil, they are preferred over roller-type bearings, particularly if a high power density at low wear is required. This is the case, for example, in bearings of machine tools, but also in the manufacture of heavy machinery where the bearings are not easily accessible and, therefore, can only be exchanged by means of expensive operations. Consequently, hydrostatic thrust bearings are frequently integrated in the drives of double-worm extruders. In these extruders, the thrust bearings primarily serve to absorb the axial pressures resulting from the extrusion process.
The load-bearing capacity of a hydrostatic thrust bearing depends on the size of the pressure surfaces and on the pressure in the foil cushion between the pressure surfaces. Consequently, there are in principle two possibilities for increasing the load-bearing capacity of thrust bearings. One possibility is to increase the oil pressure in the system and the other possibility is seen in increasing the size of the pressure surfaces. In view of the fact that the increase of the oil pressure is limited by the technology of the available hydraulic systems, increasing the size of the pressure surfaces is the only feasible possibility. However, the size of the pressure surfaces cannot also be increased indefinitely because of practical requirements, for example, in the case of shafts which are arranged parallel to each other, the radial dimension of the thrust bearing is inevitably subject to spatial limitations.
It is apparent from the above that another solution is to arrange several pairs of pressure surfaces spaced apart one behind the other in the direction of axial load application of the thrust bearings. However, this means that it must be ensured that the total load is distributed proportionally in accordance with the sizes of the individual pressure surfaces in order to avoid pressure differences between the various oil cushions. Consequently, it must be ensured that the thicknesses of the oil cushions, i.e., the gaps between oppositely located pressure surfaces of pairs of pressure surfaces, are the same in all pairs of pressure surfaces of a thrust bearing.
When using the manufacturing technologies available at present, this requirement cannot be met without significant problems when manufacturing a thrust bearing. Additional difficulties result from operationally caused different thermal and elastic deformations in the individual bearing stages, with the result that the axial spacings between the pressure surfaces of a pair of pressure surfaces vary in radial direction.
German Offenlegunsschrift 21 13 250 discloses a proposal in which the above-mentioned problems are to be solved by means of a manufacturing solution by appropriate tolerances of the gap widths of the pairs of pressure surfaces arranged one behind the other in the direction of load application of a thrust bearing. For this purpose, it is proposed to differentiate the support widths of the sliding surfaces of bearing bushings arranged one behind the other on a shaft relative to the constant support widths of the sliding surfaces of the support bushings on the abutment side, so that bearing gaps with different sizes are provided.
However, the proposal of German Offenlegungsschrift 21 13 250 has various disadvantages. One disadvantage is the fact that it is not possible to achieve a compensation between the radially inner gap width and the radially outer gap width of each pair of pressure surfaces. Another disadvantage is the fact that different gap widths in the partial load range make it necessary to use loss-producing throttles in the oil supplied to the individual pairs of pressure surfaces or oil cushions, so that discharge of the oil through the gap with the greatest width is avoided. Moreover, the proposed differentiation of the support widths requires such narrow tolerances in the micrometer range of the axial length of a pressure stage formed by a bearing bushing and a support bushing which cannot be achieved with economically acceptable costs. Also, the known proposal does not take into consideration differing temperatures of the bearing bushings and the support bushings, so that deformation differences cannot be avoided. Finally, deformation differences can also result from the use of materials having different thermal expansions.